This is an advertisement, and a poor one at that. I doubt most folks in the HN crowd would consider being owned by Automattic as a benefit, given their recent treatment of the WordPress community.

Their "click here for the thing" is broken in the dumbest way

The goods: https://github.com/Automattic/harper (Apache 2)

Also, 'grammar' in this context is English grammar, not parsers, and no other languages

It's not really clear where you download the said grammar checker or if it's open source. Looks more like a puff blog piece than something actionable.